1. Field of the Invention
The present invention relates to improved structures and processes for making the same which comprise a first structure which comprises a flame retardant polyolefin, and a second structure which comprises a polyamide which exhibits good fire retardant properties. The improved structures find particular utility as electrical insulating structures for electrical current conductors, namely wires and cables.
2. Description of the Prior Art
Multi-layer structures typically comprise at least two or more layers of materials featuring different physical properties, which properties are optimally optimized in order to satisfy a particular need at hand. The goal of the formation of such structures is to provide a structure which is directed to a particular need, and wherein the individual layers of the structures act to form a composite, and where each layer of material contributes to serve the function for which the structure is designed. Examples of such structures are well known to the art, and include composites which are used to form films, wherein each layer forming the film may feature a particular property, such as impermeability to a gas, or as a blocking layer to light or radio energy of a particular wavelength. Another example of such a structure may be those which would be used to form an insulation structure for an electrical conductor, where each layer forming a jacket about the conductor might feature improved abrasion resistance, or improved dielectric strength, or the like. Other uses of such structures are notorious and known to the art.
The use of such multi-layer structures is widespread with electrical conductors. These conductors typically consist of a wire conductor (which is frequently a metal, including copper) which is surrounded by a multi-layer structure. Examples of such constructions include those taught in U.S. Pat. Nos. 3,576,940; 3,860,686; 4,079,191; 4,292,463; 4,327,248; 4,419,538; 4,472,597; 4,510,348; 4,626,619; and 4,691,082. Therein are disclosed a variety of constructions wherein each of the layers may be a polymer, a metal, an elastomeric material, a fibrous material, etc., where each layer offers a particular advantage, i.e., dielectric strength, water impermeability, etc.
A technical paper titled "Novel Flame Retarded Moisture Curable Polyolefin Compounds" by M. J. Keogh and M. E. Lowell presented on Mar. 12-15, 1989 describes the use of various types of flame retardant polyolefins in wire and cable constructions.
Two favorable materials which may be used in the construction of electrical conductors are polyolefins and polyamides (hereinafter interchangeably referred to as "PA"). These materials are favored as the polyolefin generally forms a good flexible insulator, while PA generally exhibits good abrasion resistance properties.
While multi-layer structures used as electrical conductors are seen to perform well, such structures also suffer from certain undesirable characteristics. One such characteristic is in the fire retardancy of such multi-layer structures. In the case where such are to be used to form electrical insulation layers enrobing an electrical current conductor, such as a wire or plurality of wires, the requirement of good fire retardancy is of paramount importance. As is known, electrical cables present in a commercial, residential or other structure (i.e., office buildings, homes, public buildings, and facilities) comprise a plurality of individual wires which are contained within a jacketing layer, or other plenum. In the event of a fire within such a structure, the hazard of such a fire would be greatly compounded where the insulation layer enrobing an electrical current conducting wire were to burn or melt off, and thereby expose the wire to contact with an alternate current path. Such a current path could be the structure or element of a structure within which the wire is present, or in the more undesirable alternative, with a person's body. Further, melting of an insulating layer of a current conducting wire, particularly where proximately located to other current conducting wires with similarly degraded insulation layers raises the risk of contact between the wires and the formation of a short circuit; such an event raises the risk of ignition of surrounding materials. Alternately, if surrounding materials are not ignited, there remains the risk and consequent hazard of the ignition, burning or melting of the materials used to form insulating layers over the wire. Such materials frequently emit harmful emissions, which in sufficient concentration may prove fatal. Another disadvantage of fumes from insulation containing halogens such as chlorine found in PVC, are the harmful effects on health and on the environment. The harmful effects of even small concentrations of halogen or halogen acids is well documented. Halogen acid fumes are also very corrosive to metal. In areas where there may be articles such as instrumentation and equipment which is sensitive to the corrosive effects of acid containing or halogen containing fumes, there is the further substantial likelihood of the destruction of said articles.
Various structures providing good fire retardant characteristics are known to the art. For example, it is known to include various fire retardant additives to polyamide compositions to increase the fire retardancy thereof; such materials include halogenated organic materials in conjunction with antimony oxides. While such additives provide good fire retardancy to the polyamide, these additives frequently need be present in relatively high concentrations, typically up to 20% of the halogenated organic material, and up to 5% of an antimony oxide. A further additive known to impart good fire retardant behavior to polyamides is red phosphorous which is found to be effective in compositions at concentrations as low as 20% by weight. However, both these additive systems are also known to suffer drawbacks. Both systems are known to decrease the flexural and elongative properties of the polyamide comprising compositions of which they are a constituent. Where a polyamide containing composition having an ultimate elongation of 100% would be considered to be highly desirable, and be particularly useful in forming part of a flexible structure well suited for insulating electrical current conductors, the use of such types of a additives are also known to substantially reduce the ultimate elongation and other flexural characteristics of polyamide compositions of which they comprise a part. Additionally, red phosphorous is known to impart a deep color to polyamide compositions which is also frequently undesirable.
It should be apparent that there remains a continuing need in the art for multi layer structures featuring good flexural characteristics and good fire retardant properties, as well as methods for making the same. Such multi layer structures would be particularly useful in forming insulation structures for electrical current conductors.